1. Field of the Invention
The invention relates to a record carrier for thermomagnetic recording of information and magneto-optical read-out of the recorded information.
2. Description of the Related Art
Thermomagnetic recording of information is a known recording method which is disclosed, for example, in Philips Tech. Rev. 42, No. 2, pp. 51-58 (1985). In this method a record carrier is used which has a substrate and a thermomagnetic recording layer thereon. To record information, the record carrier is exposed to a light beam originating from, for example, an AlGaAs laser, having a wavelength of approximately 820 nm. The laser beam is focussed on the recording layer by means of a lens system. Such layer includes a magnetic recording material having a perpendicular magnetic anisotropy, the easy axis of magnetization being perpendicular to the layer surface. In the areas which are exposed to the laser beam the temperature of the magnetic material increases, and subsequently the direction of magnetization of the heated areas is reversed. Such reversal can take place spontaneously in response to interaction with the magnetic field of the adjacent magnetic material.
Preferably, however, the direction of magnetization is reversed by application of an external magnetic field whose field direction is opposite to the original direction of magnetization of the recording layer. After exposure, the magnetic material cools and the reversed direction of magnetization becomes fixed, the areas having reversed magnetization being representative of the recorded information. This information can be read by means of a beam of polarized laser light, on the basis of magnetic rotation of the polarization plane of such beam. This rotation is known as the Kerr-effect in the case of reflection and as the Faraday effect in the case of transmission of polarized light.
So far only two classes of magnetic materials are known in which thermomagnetic recording of information is possible. Although many magnetic materials are known which evidence perpendicular anisotropy, only a few of these, belonging to said two classes, have been found to be suitable for thermomagnetic recording.
This is not very surprising since for thermomagnetic recording the properties of the material must satisfy very severe requirements. In addition, these requirements are often of opposite nature; that is to say, satisfying one means that satisfying the other is more difficult. The various properties which the thermomagnetic recording material should have are
a) a perpendicular magnetic anisotropy
b) a rectangular BH hysteresis curve (i.e. a remanence of 100%), with high coercive force H.sub.c at ambient temperature. The H.sub.c value must exceed the writing field strength, i.e. larger than 40 kA/m,
c) a high magneto-optical figure of merit, R.theta..sup.2, wherein R represents the reflectivity and .theta. the rotation of the polarization plane of light on interaction with the recording material,
d) a relatively low Curie temperature T.sub.c,
e) a magnetic switching characteristic such that switching of the direction of magnetization can be effected at various powers of the write beam and at minimal strength of the (external) magnetic field used, preferably less than 40 kA/m,
f) low disk noise and write noise,
g) be properly workable at low temperature, and
h) appropriate physical and chemical stability.
The two classes of materials which reasonably satisfy these requirements are
1. The class of amorphous rare earth metal and transition metal alloys, such as those described, inter alia, in Appl. Phys. Lett. 22 337 (1987). Well-known and properly efficient materials of this class are, for example, GdTbFe or TbFeCo.
2. The class of oxidic compounds. In this class a further distinction can be made between (mono) crystalline garnets and ferrites. Thermomagnetic recording employing monocrystalline garnet layers is disclosed, inter alia, in J. Appl. Phys. 36, 1110 (1965). The use of ferrites is described in U.S. Pat. No. 4,586,092.
The prior art thermomagnetic recording materials described above have various disadvantages.
Ferrites have the drawback that the noise introduced by this material is relatively high. This causes a low signal-to-noise ratio (SNR), so that these materials are not so suitable for the recording of, for example, video (image) signals. A further drawback is that the ferrite layers must be processed into a recording layer at an elevated temperature. Thus, these materials are deposited on a substrate by, for example, sputter deposition at 400.degree.-500.degree. C. This means that the substrate must be capable of withstanding such high temperatures. It is therefore not feasible to use a synthetic resin substrate or a substrate coated with a synthetic material layer. But it is precisely the use of a synthetic resin substrate or a substrate coated with a synthetic resin layer which is of great practical importance, because it is possible to form a guide track, for example a helical groove, in a simple and cheap manner in such a synthetic resin. By means of the guide track the laser light beam is guided and controlled during the writing or reading process.
Monocrystalline rare earth metal containing garnets have the disadvantage that their production is very expensive. In practice, monocrystalline layers of this type are only marginally suitable for thermomagnetic recording, and actually only for special, professional applications. The substrate onto which such a monocrystalline layer is provided must be a non-magnetic, monocrystalline garnet material. Also in this case there is the above-mentioned drawback that the use of substrates containing a synthetic resin material is excluded. In addition, these materials have a very high transmission, so that energy transfer from the write laser beam is very low.
Thermomagnetic recording layers of GdTbFe or TbFeCo have up to now proved to be the most promising materials, but have the important disadvantage of strong susceptibility to corrosion (oxidation). As a result, the layers become unsuitable for recording after a short period of time and, in addition, the material already recorded is lost. To obviate this drawback it has been proposed to provide protective layers, but this only partially alleviates the corrosion problem. Moreover, this additionally complicates the structure of the record carrier, which consequently becomes more expensive.
A further disadvantage of the use of rare earth metal and transition metal alloys for thermomagnetic recording is the magnitude of the magneto-optical effect of this material. Magneto-optical effect is here to be understood to mean the rotation of the polarization plane of the polarized laser beam used for read-out. As will be described in more detail hereinafter, the magnitude of this effect plays an essential role during read-out of thermomagnetically recorded information. It has been found that the magnitude of this effect, unfortunately, decreases at shorter wavelengths of the read beam. More specifically, for a wavelength shorter than 820 nm, which at present is that most used for the read beam. If shorter wavelengths could be used, for example produced by what is commonly denoted a blue laser, then writing and/or reading could be effected with a higher recorded information density.